Vanadium was once considered an oddity in biology with ill defined structures and even more obscure functions. In the past two decades, scientists have realized that this element is found in marine enzymes that may be responsible for many useful natural products and ultimately may serve as a new therapeutic regiment for oral treatment of diabetes. During our previous three years of funding, we have focused on preparing functional models for the vanadium haloperoxidases. Early last year, we presented the most efficient haloperoxidase reactivity model compounds yet reported. Unlike previous models, our compounds are functional as mononuclear, monoperoxo species, exactly as proposed for VHPOs. About six months ago, we made a major breakthrough in that we can oxidize chloride at a measurable rate. We estimate that our reaction is at least 10 (and more likely 100 fold) faster than any previous vanadium based chloride oxidation catalyst. We also now have data that may address the functional role of amavadin which we have suggested may be a rudimentary bromo- or iodoperoxidase. The studies proposed for the next funding period build on these major advances. 1) Probe the mechanism of vanadium haloperoxidases by establishing the factors that lead to activation of our bromide and chloride oxidation catalysts. As necessary, we will develop and characterize new ligand sets that serve as more efficient agents in these reactions. 2) Expand our studies of the reaction of V(IV) species with hydrogen peroxide since these may be important interaction that define the reactivity of amavadin and the cellular metabolism of vanadium insulin mimics. 3) Through collaboration with several spectroscopists, develop new methodologies that will allow scientists to define to higher chemical resolution V=O binding sites in proteins. Since V=O substitutes for many divalent ions, this approach can be broadly applied to a variety of metalloproteins. We propose that high molecular weight acid phosphatases can convert to VHPOs in the presence of vanadate.